A triangle wave comparison PWM control method has been generally known as the motor control apparatus and motor control method described above. In this control method, a command voltage for each phase is calculated based on a difference between a current command value and a current flowing through each phase, and a switching mode of an inverter is determined based on whether the calculated command value is larger or smaller than a carrier having a triangle wave shape.
As disclosed in, for example, a patent document 1 and a patent document 2, a technique for improving current responsivity in this triangle wave comparison PWM control method has been known.
In a control method disclosed in the patent document 1, a future current (torque) in each switching mode of an inverter is predicted by using a motor model based on a current flowing through a motor, a position of a magnetic pole of a rotor, and a rotation speed of the rotor. Then, a switching mode is selected so that a difference between the future current (torque) and a current (torque) command value for controlling the motor can be minimized, and the inverter is driven in the switching mode.
In the patent document 2, to perform the model prediction control disclosed in the patent document 1 more accurately, a predicted current ide (n+1), iqe(n+1) one control cycle ahead is calculated based on a present voltage vector (switching mode) V(n) of an inverter. Then, a predicted current ide (n+2), iqe(n+2) further one control cycle ahead is calculated for each voltage vector V(n+1) by using the predicted current ide (n+1), iqe(n+1) as an initial value. Then, by using an evaluation function J to which the predicted current ide (n+2), iqe(n+2) is inputted, a voltage vector V(n+1) one control cycle ahead is determined so that a difference from a command current can be minimized.
In the techniques disclosed in the patent document 1 and the patent document 2, a future current which will flow through a motor is predicted for each switching mode (each voltage vector) of an inverter, and a switching mode is selected so that a difference between the future current and a command current can be minimized.
Therefore, there is a problem in that a calculation processing load for determining the switching mode for driving the motor is increased. In particular, this problem will be more serious if the motor model is constructed as a complicated mode in consideration of an inductance L, a non-linearity of the number of flux linkages φ, and a disturbance.
Further, there is a need that the calculation processing is completed within a control cycle of the motor. It is preferable that the control cycle of the motor be set to a few microseconds to be equivalent to phase resolution of the triangle wave comparison PWM control method. When the calculation processing is implemented by hardware, serial processing cannot complete the calculation processing in time. Therefore, there is a need to perform parallel processing using a lot of processors or prepare a multidimensional state-space map (switching mode table) for determining a switching mode based on inputted state quantities such as a rotation speed and current. As a result, there arises a problem in that a circuit size is increased. In contrast, when the calculation processing is implemented by software, a high-performance microcomputer with high processing speed is needed. As a result, there arises a problem in that a cost is increased.